1. Field of the Invention
The invention relates to a method for the preparation of N-cycloalkylideneanilines by elimination of water catalysed by acid and by azeotropic removal of the water of reaction.
The method comprises the reaction of optionally ring-substituted anilines with optionally substituted cycloalkanones in the presence of at least one acid catalyst which is soluble in mixtures of aniline/cycloalkanone/azomethine, at a raised temperature in a distillation column.
The condensation of aromatic amines with cycloalkanones is an equilibrium reaction with a small heat of reaction. It is shifted towards the azomethine (Schiff base) if the water generated in the reaction is removed from the system. In general this is achieved by azeotropic distillation, an inert entrainer such as benzene, toluene etc. being used if required.
2. Description of the Related Art
More recently, the binding of the water of reaction by using TiCl.sub.4 (J. Org. Chem. 32 (1967), 3247) or molar quantities of (butyl).sub.2 SnCl.sub.2 (Synth. Commun. 12 (1982), 495) has been described. Both these compounds bind the water of reaction produced while liberating HCl.
In other developments, the species binding the water is bound covalently to the aniline, for example in the form of N,N-bis-(trimethylsilyl)-aniline (Bull. Soc. Chim. Fr. 1966, 3205), iminophosphoranes (Angew. Chem. Int. Ed. Eng. 5 (1966), 947) and N-(diphenylaluminium)-aniline (J. Org. Chem. 51 (1986), 1848).
A further method of binding effectively the water of reaction produced and thus being able to work under mild conditions, using small excess amounts of aniline or ketone, is to use molecular sieves (J. Org. Chem. 36 (1971), 1570; German Offenlegungsschrift 2 244 238). The disadvantage of this last-mentioned method is the laborious and expensive regeneration of the molecular sieve.
Azeotropic dehydration is certainly the method of greatest industrial interest if it is possible to carry out the reaction with low energy expenditure, a good space-time yield and selectivity, and without excessive quantities of azeotropic entrainer in a simple apparatus.
High space-time yields require as a precondition a high reaction rate, and therefore effective catalysis.
The condensation of cyclohexanone and aniline to give cyclohexylideneaniline, using zinc chloride as a catalyst, has long been known (Ber. 53 (1920) 345-354). In difficult cases of condensations of this type, the catalyst system HCl-ZnCl.sub.2 has been used (Ber. 46 (1913) 2718). German Auslegeschrift 1 078 119 describes the condensation N-phenyl-p-phenylene-diamine with cyclohexanone without the addition of a catalyst; in such cases the use of a cyclohexanone excess of 200 to 300% is required.
German Offenlegungsschrift 2 525 295 discloses that the reaction time of the condensation of aniline, using a 400% excess of cyclohexanone without a catalyst, increases sharply with increasing batch size, making it impossible to scale up to industrial level. It was also disclosed that strongly and weakly acid organic resins affect the reaction time favourably.
German Offenlegungsschrift 2 901 863 describes freshly synthesised, anhydrous, non-calcined calcium hydrogen-phosphate, apatite of the formula Ca.sub.5 (PO.sub.4).sub.3 OH, dried, non-roasted aluminium oxide hydroxides and proton-exchanged aluminium silicates washed to neutrality of the montmorillonite type as effective catalysts for the reaction of aromatic amines with ketones. The examples in this patent application are limited, however, to the condensation of the reactive p-phenylenediamine with methyl isobutyl ketone, a 150% excess of ketone being used.
All the abovementioned methods for azomethine synthesis by azeotropic dehydration have the drawback that it is both time-consuming and energetically expensive to take the reaction to completion.
It was therefore desirable to develop a cost-effective and environmentally friendly method for the synthesis of cycloalkylideneanilines, which method is to be notable for high yields, simple apparatus and optimum energy utilisation with minimum energy consumption.